Neurotensin has been shown to cause histamine secretion from mast cells, to elevate blood levels of histamine, and to bind specifically to synaptic membrane in brain. Various other neuropeptides have been shown to affect neuronal activity of individual nerve cells. This research project will studythe influence of this neuropeptide on the functioning of a histaminergic neuron. The large, identified histaminergic neuron of Aplysia californica will be used as a preparation. Postsynaptic potential changes and spike activity in the varous follower cells will be monitored using intracellular recording techniques. The effects of neurotensin will be determined by monitoring changes in thse parameters. Confirming evidence for the histaminergic nature of the neuron will come from measurements of histamine levels within the neuron, histidine decarboxylase activity inthe neuron, and the use of the iontophoretic application of histamine and its antagonists.